1. Field of the Invention
The present invention relates in general to the control of a power relay for actuating a magnetron of a microwave oven, and more particularly to a method for controlling the power relay in which contacts thereof are closed at various points of time or phase of an alternating current (AC) waveform input thereto with an equal probability, thereby preventing a transfer of the contacts of the power relay.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional relay control circuit for a microwave oven. As shown in this drawing, the conventional relay control circuit comprises a microcomputer 2 for controlling components of the microwave oven, a power supply 3 for supplying power to the components of the microwave oven as a power cord of the microwave oven is connected to a power source, a main relay 4 for actuating an oven lamp 6, a fan motor 7 and etc., a power relay 8 for actuating a magnetron, a relay driver 5 for driving the main relay 4 and the power relay 8 under the control of the microcomputer 2, and a key input unit 1 for inputting information about cooking which is required by the user.
In operation, first, the power supply 3 supplies a standby power to the components of the microwave oven as the power cord is plugged in the power source. The microcomputer 2 is energized by the standby power from the power supply 3, so as to scan a key input signal from the user. When the user pushes a start key after he places food to be cooked in a heating chamber (not shown) of the microwave oven and finishes inputting key inputs, the microcomputer 2 controls the relay driver 5 to drive the main relay 4 so that the oven lamp 6 and the fan motor 7 can be actuated. The microcomputer 2 then controls the relay driver 5 to drive the power relay 8 so that the magnetron can be actuated.
At this time, if contacts of the power relay 8 continue to be closed at a particular phase of an AC sinusoidal waveform, a transfer of the contacts of the power relay 8 may occur. As a result, the contacts of the power relay 8 are subjected to a damage due to the transfer. For this reason, it is preferred to make the contacts of the power relay 8 closed at, various phases of time or phase of the AC sinusoidal waveform with an equal probability.
For the purpose of preventing the transfer of the contacts of the power relay, there have conventionally been proposed a method for actuating the magnetron of the microwave oven using a triac instead of the power relay.
FIG. 2A is a waveform diagram illustrating the conventional method for actuating the magnetron of the microwave oven using the triac. In this case, the magnetron is actuated at the peak phase of a voltage of an AC sinusoidal waveform which is applied into the microcomputer for time check. The reason is as follows. A current lags behind a voltage by .+-..pi./2 in phase, since the power of the AC sinusoidal waveform is applied to the magnetron through a high voltage transformer (HVT). As a result, the current becomes "0" at the point of time that the voltage becomes the peak. This helps minimizing an inrush current. Further, the magnetron can be actuated at the same phase that the power relay is driven, because the sinusoidal waveforms applied to the microcomputer and to the power relay are different in amplitude but the same in phase. Also, since switching time of the triac is several [ns] which is very shorter than that of the power relay, the magnetron can be actuated accurately at the point of time that the voltage becomes the peak.
However, the above method using the triac has a disadvantage, in that it is not economical since the triac is very expensive. Also, a separate cooling system must be provided because a consumption power of the triac is commonly 1300 W or above.
FIG. 2B is a waveform diagram illustrating another conventional method for actuating the magnetron of the microwave oven using the power relay, with the purpose of preventing the transfer of the contacts of the power relay. This method is a positive-negative random crossing method which controls the point of time that the power relay is driven, in such a manner as (+) 1.fwdarw.(-) 2.fwdarw.(+) 3.fwdarw.--(-) 8.fwdarw.(+) 1--at random phases of the sinusoidal waveform applied to the microcomputer. The purpose of this method is to prevent the contacts of the power relay from being damaged due to the transfer in the case where the contacts of the power relay continue to be closed at a particular phase of the sinusoidal waveform. In this case, an interval between the adjacent power relay driving points of time can be obtained by controlling the phase of the AC sinusoidal waveform applied to the microcomputer. Assuming that a frequency of the AC sinusoidal waveform applied to the microcomputer is 60 Hz and the time of a period is thus 1/60 sec, the interval between the adjacent power relay driving points of time is 1/480 sec (1/60.times.1/8) since a period includes 8 of the power relay driving points of time as shown in FIG. 2B. As a result, the power relay driving point of time is delayed by 1/480 sec with respect to the positive and negative phases of the AC sinusoidal waveform.
However, the above method using the power relay is disadvantageous in that the cost is increased since a control program is very complex, resulting in an increase in a size of a ROM.